As an example of the influence of changes in the light environment in the nature on the immune systems of living organisms, it has been reported that there is recognized a negative correlation between the degree of the unpleasant mood or behavior disorder due to decreased sunshine duration in autumns and winters and the number of natural killer cells (hereinafter referred to as NK cells) in the peripheral blood (see Non Patent Literature 1 below). This report suggests the possibility that continuous light such as sunlight in the daytime may be effectively used for immune regulation. Regarding reactions in the bodies of living organisms to light, it has been said that in birds, reptiles, and fish, for example, light passes through the cranial bones other than the visual pathways and thus directly reacts with the photosensitive cells in the pineal gland. Meanwhile, such reactions have not been known for humans.
When medical effectiveness of light including those other than natural light is considered, medium-wavelength (UVB) or long-wavelength (UVA) ultraviolet rays have been applied to the treatment of skin diseases, such as psoriasis, vitiligo, and atopic dermatitis, while bright light therapy has been applied to the treatment of seasonal affective disorder (SAD), depression, and the like. Meanwhile, near-red light beams have been applied to the treatment of pain, skin ulcer, and the like. Thus, light has been already applied widely, in particular, to clinical medicine. Regarding a medical report of the use of diode light for a light source, basic research that recognized the enhancement of the wound healing effect with the use of light of near-red light-emitting diodes has been conducted so far. Medical usability of light sources with low invasiveness has also been researched.
There have been a number of reports of research on the influence that visible light has on the immune response via the visual pathways, that is, the nervous system. In addition, the inventor has already reported that the activation of α waves in the frontal region that is seen when a healthy subject is optically driven is correlated with the activation of the cell-mediated immunity in the peripheral blood.
However, photic stimulation that is applied via the eyes through optical drive can become a sort of physical stress on the subject. Therefore, it has been considered that such photic stimulation prevents the enhancement of the immune system.
Further, the inventor has proposed technologies for non-invasively enhancing the activity of the brain and immune system by irradiating the head with a light beam from the side of the frontal region while completely shielding the both eyes from the light beam (see Patent Literature 1 and 2).
An irradiation tool described in Patent Literature 1 includes a light source portion adapted to be in contact with the frontal region, and a band portion for mounting (fixing) the light source portion on the head of the user. The inner side of the light source portion (the side of the frontal region of the user) has a number of LEDs arranged therein. A light-shielding portion is provided on the lower edge portion and the side edge portion of the light source portion.